The present invention relates to a liquid filter of the type filled with granular material wherein a liquid to be processed flows downwardly through totally immersed granular material, during which the liquid to be processed is placed in contact with a gas which circulates upwardly through the granular material.
This type of filter is employed for processing various types of water, and in particular for achieving nitrification of water, a process which consists of causing oxidation of organic and ammoniacal nitrogen into nitrites, and then into nitrates, through the action of specific bacteria. Since these bacteria grow only in aerobic media, the nitrification process requires dissolving in the water to be processed a certain quantity of oxygen which is a function of the flow of water to be processed and of the quantity of nitrogen to be oxidized. The required quantity of oxygen, originating for instance from the atmospheric air, therefore is introduced into the filter. In general, the air is injected from the bottom to the top of the filter through the granular material of the filter.
Because of the bacterial growth inherent to this type of nitrification process, a clogging of the granular material eventually occurs, and the granular material accordingly eventually must be cleaned by a washing operation involving water and air.
This type of filter, filled with granular material, normally is in the form of an enclosure structure, for example of cement, which is upwardly open and which includes a bottom and a floor positioned above the bottom, the floor supporting the granular filter material. The granular filter material is immersed in the liquid to be processed, such that the liquid to be processed circulates downwardly through the bed of granular material and is discharged below the floor. The floor is fitted with nozzles which allow for the introduction of water and air for washing the filter by circulation from the bottom to the top through the filter material, when it becomes necessary to undergo a washing operation.
In this type of filter, conventionally the gas required for development of the nitrification process is distributed through a system of perforated tubing positioned above the floor in the area of the nozzles through which the water that has flowed downwardly through the granular material is drained. This type of conventional arrangement has a number of inherent disadvantages. The construction is complicated. A network of tubing must be placed within the granular material. Since this network of tubing is subject to a degree of stress which varies depending on the amount of clogging within the granular filter material, it becomes necessary to support the network of tubing. Distribution of the nitrifying gas through such a network of tubing can be properly achieved only if the flow rate of the gas does not vary substantially from the initial value taken into account during calculations for the filter device. It is apparent that for the system to be economical, the gas flow must be adjusted to the flow of water to be processed and to the quantity of nitrogen to be oxidized, both of which are variable parameters of the particular processing installation. Such an adjustment, which must not be detrimental to the qualitative results of the process, can be obtained only if the distribution of gas in the granular filter material remains accurate when its flow decreases. Otherwise, anaerobic areas would form locally within the granular filter material, and such anaerobic areas would be harmful to the living nitrifying bacteria and thus detrimental to the quality of the processing operation. Moreover, as the bacteria growth would be arrested in such anaerobic areas, such areas would form locations of minimum clogging through which the water to be processed would have a greater tendency to pass. Further, the presence of the network of tubing above the floor supporting the granular material disturbs the flow of fluids during a washing operation and thus is detrimental to the efficiency of the washing operation.